Automatic dishwashing (ADW) detergents constitute a generally recognized distinct class of detergent compositions whose purpose can include to break down and remove food soils; to inhibit foaming; to promote the wetting of wash articles in order to reduce or eliminate visually observable spotting and filming; to remove stains such as might be caused by beverages such as coffee and tea or by vegetable soils such as carotenoid soils; to prevent a buildup of soil films on wash ware surfaces; and to reduce or eliminate tarnishing of flatware without substantially etching or corroding or otherwise damaging the surface of glasses or dishes. The problem of glassware corroding during washing in an automatic dishwashing appliance has long been known. Current opinion is that the problem of corrosion in glassware is the result of two separate phenomena. On one hand, the high pH needed for cleaning causes silica hydrolysis. This dissolved silica/silicate, together with silicates added purposely to prevent china and metal corrosion, deposit on the glassware surface leading to iridescence and clouding. On the other hand, builder removal of chelate metal ions from the glassware surface, and the subsequent metal ion leaching that follows renders a less durable and chemical resistant glass. After several washes in an automatic dishwashing appliance, both phenomena can cause damage to glassware such as cloudiness, scratches, and streaks. This can happen in both the main wash (or through-the-wash) cycles, as well as, in the rinsing/drying cycle.
Most consumers agree that corrosion of glassware from use of detergent compositions in automatic dishwashing (ADW) is one of their most serious unmet needs. ADW detergent compositions containing zinc or magnesium salts of organic acids for improved protection against glass corrosion are known. As these salts are sparingly soluble, they are used for controlled release of reactive zinc species. The use of soluble zinc salts in detergent compositions is difficult to control as precipitates of insoluble zinc salts with other ions in the wash liquor will occur. Yet insoluble zinc salt precipitates may deposit on both the glassware and on the ADW appliance elements itself. Furthermore, some insoluble zinc salts may be too inert to deliver the needed Zn2+ ions, as for example zinc oxide (ZnO). Aluminum sulfate salts have also shown promise, but formulatabilty issues remain. For example, flocculation with a polymer thickener and a slight negative on oxygen bleach performance requires an encapsulation approach, which can add formulation costs.
In rinse aid applications, compositions comprising water-soluble metal salts (such as zinc salts of chloride, sulfate or acetate) for use in automatic dishwashing afford some measure of glassware protection. Water-soluble zinc salt may be employed to prevent the corrosion of ceramic surfaces. Solid metal plates of zinc alloys may also be used in combination with a detergent composition to provide corrosion protection to glassware. A water-soluble zinc salt may even be used in conjunction with a low-foaming nonionic surfactant in neutral to high pH. However, the use of this high pH composition in automatic dishwashing can result in unsatisfactory filming and precipitation of insoluble materials. Such precipitant material is very undesirable as it can adhere to internal dishwasher parts, as well as, onto dishware and glassware during the washing cycle. One alternative using soluble zinc and a chelant provides some glassware corrosion protection but has a filming negative (i.e. crystals and films formed on glassware). Yet another alternative is to use insoluble zinc salt to control the release of the Zn2+ ions in the rinse to avoid filming. However, there are disadvantages of using insoluble materials in the liquid rinse aid formulations. The product would be cloudy and it requires particular thickeners and stabilizers that may hinder delivery of the product from the rinse aid dispenser to the rinse liquor.
Since glass corrosion occurs in both the wash and rinse/drying cycles, there is a continuing need to develop improved complete-cycle, ADW methods using both a through-the-wash (TTW) detergent composition and a rinse aid composition so that the problem of glassware corrosion is reduced versus using either compositions alone.